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In perusing the settings on my EVO today, I noticed that there is an option under call settings named "Voice Privacy". The description beneath the checkbox merely states "Enable enhanced privacy mode":

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The screenshot is from Deck's ICS pre-beta, but I'm fairly positive I saw this in CM7 as well because it seems very familiar. I've also seen this settings on my Verizon Galaxy Nexus running both ICS (stock and CM9) and Jelly Bean.

Both phones I've seen this on were CDMA devices - an HTC EVO on Sprint, and a Verizon Galaxy Nexus. What little information I've been able to dig up seems to suggest that it may be unique to CDMA, but I cannot say that for certain.

What does this setting do? Some theories I've found on forums have included reducing the earpiece volume (so others can't overhear your conversation, I suppose?) and adding extra encryption to the CDMA connection, but I haven't found anything definitive.

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3  
This setting is not available on my CM7 GSM device (Samsung Galaxy S Vibrant). edit: I found one place that said: 'it automagically translates your voice into Kilngon for transmission' - I want to think that it does this... –  Ryan Conrad Mar 23 '12 at 17:27
    
Haven't ever seen this on any of my devices. Do you have a CDMA phone or a GSM one? –  ce4 Jul 2 '12 at 20:51
    
@ce4: Both phones I saw this on were CDMA. First was an HTC EVO (Sprint), now I have a Galaxy Nexus (Verizon). –  eldarerathis Jul 2 '12 at 20:58

3 Answers 3

up vote 8 down vote accepted
+100

This is a feature of CDMA (standardized in IS-95) and is called Voice Privacy.

See an Analysis of IS-95 CDMA Voice Privacy by M.Zhang, et al. from 2000, free download here

Citation (the real paper begins at p.10 in the PDF:
Abstract. The voice privacy of IS-95 CDMA cellular system is analyzed in this paper. By exploiting information redundancy on the downlink traffic channel, it is shown that an eavesdropper can recover the voice privacy mask after eavesdropping the transmission on the downlink traf- fic channel for about one second. Thus, IS-95 CDMA voice privacy is vulnerable under ciphertext-only attacks.

That cryptanalysis is now 12 years old and already then was the scheme considered broken. I guess it's easy to suggest to just leave the setting disabled.

By combining some information grepped from the android source, I'm pretty sure it's what you're asking for.

To make it a little more transparent here are some source references:

packages/apps/Phone/res/xml/cdma_call_privacy.xml defines:

<?xml version="1.0" encoding="utf-8"?>
<PreferenceScreen xmlns:android="http://schemas.android.com/apk/res/android"
        xmlns:settings="http://schemas.android.com/apk/res/com.android.phone"
        android:title="@string/additional_cdma_call_settings">

    <com.android.phone.CdmaVoicePrivacyCheckBoxPreference
        android:key="button_voice_privacy_key"
        android:title="@string/voice_privacy"
        android:persistent="false"
        android:summary="@string/voice_privacy_summary"/>
</PreferenceScreen>

packages/apps/Phone/res/values/strings.xml defines those strings:

<string name="voice_privacy">Voice Privacy</string>  
<string name="voice_privacy_summary">Enable enhanced privacy mode</string> 

./hardware/ril/include/telephony/ril.h defines also:

typedef struct {
    RIL_CallState   state;
[...]
    char            isVoice;    /* nonzero if this is is a voice call */
    char            isVoicePrivacy;     /* nonzero if CDMA voice privacy mode is active */
[...]
} RIL_Call;
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For the record: I did compile this answer (longest part was to include and reference all the grepped stuff) while ryan was a little faster to post. Haven't seen his answer until after the post though. :-) –  ce4 Jul 2 '12 at 21:49
5  
sorry for the dev stuff - Don't be! If you can support your answer with Android source code then I say by all means, do it! Providing a "layman" summary is helpful for non-programmers, but if you look at some of my posts I think you'll find I'm certainly not opposed to delving into AOSP to find answers :) –  eldarerathis Jul 3 '12 at 0:06
    
Removed. Thanks for the hint! "Delving into AOSP to find answers": absolutely correct. That's where the ultimate answers lie, it's the only way to be sure (TM) sometimes :-) –  ce4 Jul 3 '12 at 8:12
1  
Use the source luke :P –  t0mm13b Jul 3 '12 at 23:38

From the My HTC EVO 3D book:

Touch to enable voice privacy, which makes your EVO 3D encrypt your phone calls. Enable Voice Privacy

What is Voice Privacy?

When you enable Voice Privacy, your EVO 3D starts encrypting your calls so nobody can eavesdrop on them. ... By encrypting your phone calls between your EVO 3d and the cell tower, you add an extra layer of protection on your calls just in case someone wants to try to listen in. For this feature to work, your cellular service provider must support it, otherwise enabling it on your EVO 3D is useless.

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Voice Privacy is something that is part of CDMA. I found this article that talks a little bit about it in the introduction.

Voice privacy of IS-95 CDMA is provided by means of the long code mask.The long code mask is not transmitted through any channel, it is constructedby the base station and the mobile station. To recover the long code sequence,the eavesdropper may exhaustively search the 42-bit long code mask, with atime complexity of O(2 42 ). This attack is viable but is hard to implement inreal time. Alternatively, it can be shown that the long code sequence can also be recovered if the eavesdropper can obtain 42 bits of plaintext-ciphertext pairs.As there are many mobile stations transmitting simultaneously on the tra cchannel and each mobile station only transmits approximately 3 minutes onthe average, it is rather di cult to obtain 42 bits of the plaintext message.

I have also found a couple posts that talk about how sprint did studies that said it isn't really any more secure, unless your eavesdropper has tons of money to spend on decrypting your call.

Actually, voice privacy on CDMA is an added layer of voice encryption. Sprint did some independent research (I've heard quotes as big as a million dollars worth) and determined that CDMA without Voice Privacy is just as secure (unless you've got a million dollars worth of financing to break into an individual phone call)...

In other words, turn the feature off, and don't worry about it.

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"In other words, turn the feature off, and don't worry about it." -- what's the disadvantage of leaving it turned on? –  Erik Jul 2 '12 at 21:58
    
It's fundamentally broken. Period. Researchers have 'shown that an eavesdropper can recover the voice privacy mask after eavesdropping the transmission on the downlink traf- fic channel for about one second'. –  ce4 Jul 2 '12 at 22:11
    
Am curious, as coming from EU, with GSM, why is that in there in the first place? –  t0mm13b Jul 2 '12 at 22:12
    
It's just a feature of the competing CDMA standard and because it's optional, someone chose to make it an option also in android. However: GSM is also fundamentally broken. See youtube.com/watch?v=rl5uq7EzVYQ for more. –  ce4 Jul 2 '12 at 22:21

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